Liquid and gas chromatographs are widely used to measure the relative concentrations of chemicals in a mixture. They are extremely sensitive. In operation, a small sample of the mixture to be analyzed is injected into a steady flow of carrier medium just as it enters a long thin tube known as a column. Due to action of material in the column, each chemical emerges from the column during a different interval following the time of injection. Inasmuch as the time it takes each chemical to pass through a given column is known from previous experiment, the identity of a chemical emerging during a given interval is established.
A detector that produces an electrical signal proportional to the intensity of a characteristic of matter flowing through it is connected to receive the effluent from the column. The carrier medium is selected so as to have a significantly different intensity of the characteristic per unit mass than the chemicals being analyzed. Accordingly, when the effluent from the column consists of pure carrier medium, the signal produced by the detector has a predetermined value termed the baseline. During the intervals when the effluent from the column contains one of the chemicals being measured, the signal gradually moves above or below the baseline in the form of a peak. The area between the peak and the baseline measured by an integrator and is proportional to the amount of the chemical in the sample.
The temperature of the column is controlled in an oven. The material contained in the column bleeds, i.e., it puts chemicals of its own into the flow of carrier medium. If this occurs at a steady rate, as it does when the temperature of the column is kept steady, the only effect is to shift the baseline by a constant amount and does not present a problem. This is known as isothermal operation.
However, when certain chemicals are being analyzed, optimum separation between the intervals during which they emerge from the column is only attained by programming the temperature of the oven. This changes the amount of column bleed and causes the baseline to drift by varying amounts so that it is difficult for the integrator to determine the area of a peak that is due to the presence of a chemical being measured.
For as long as fifteen years the solution to this problem has been the use of what is known as dual column operation. Two columns are mounted in the oven and a separate detector is provided for each. Equal flow of identical carrier medium is introduced into each column but the sample of chemicals is injected into only one. The amount of carrier medium and column bleed from each column is ideally the same. If the detectors are identical, the baseline signals of each are the same and the only difference in the signals provided by the detectors is due to the presence of the sample of chemicals being analyzed. Therefore, if the signals from the detectors are subtracted, a signal is derived that represents the concentrations of the various chemicals in the sample.
Whereas this method tends to cancel the deleterious effects of column bleed, it makes no correction for the differences in the responses of the detectors employed. This is especially important when it is desired to use a thermal conductivity detector because in spite of great expense and care in manufacture as well as selection, it is difficult to attain a precisely matched pair of detectors. Even if initial match is achieved, it is generally lost during operation or for that matter while the detectors are sitting on the shelf due to ageing. In an effort to provide matched pairs of thermal conductivity detectors, both have been inserted into a single large specially constructed metal block. Before accurate results can be attained, all parts of the block must reach the same temperature, and this requires as much as twelve hours.
The thermal conductivity detector has many advantageous characteristics, including low cost, but it is so adversely affected by ambient conditions, in spite of the large block, that its use is limited.